There has hitherto existed a printed board in which a circuit composed of a copper foil is formed on a substrate by photolithography. Photolithography includes the step of etching a copper foil and high costs are required for a treatment of waste fluid generated by etching.
There has been known, as the technology requiring no etching, a method in which a copper particulate dispersion prepared by dispersing copper particulates (copper nanoparticles) in a dispersion vehicle (in a vehicle) is discharged from an ink-jet printer on a substrate in the form of droplets to form a circuit (see, for example, Patent Literature 1). According to this method, a film of the copper particulate dispersion is formed on the substrate by discharging droplets and, after drying the film, copper particulates in the film are melted by exposure to light and thus conductivity is imparted to the film. It is considered that the viscosity of the copper particulate dispersion is desirably less than 20 mPa·s so as to enable the dispersion to discharge in the form of droplets. The film of the copper particulate dispersion is dried by heating to room temperature or a temperature of 150° C. or lower so that a substrate is not damaged by heat even if the substrate is made of a resin. For the purpose of making an attempt to reduce the amount of the dispersion vehicle which remains after drying, a dispersion vehicle having a boiling point of lower than 150° C. is selected.
However, because of low boiling point of the dispersion vehicle, such a copper particulate dispersion is likely to be excessively dried to cause clogging with copper particulates at the portion from which the dispersion is discharged in the form of droplets. In contrast, high boiling point of the dispersion vehicle may sometimes cause an increase in viscosity, and thus it may become difficult to discharge the copper particulate dispersion in the form of droplets.